.‘With,  tile  compliments  oi  tlie  autnor 


«  J 


If 


[From  The  American  Journal  of  Science,  Vol.  XLII,  September,  1916] 


NEW  POINTS  on  the  ORIGIN  of  DOLOMITE. 


By  Francis  M.  VanTuyl. 


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YanTuyl — New  Points  on  the  Origin  of  Dolomite.  249 


Art.  XXVIII. — New  Points  on  the  Origin  of  Dolomite  :*  by 

Francis  M?  V  anTuyl. 
m 

Historical  Review. 


The  problem  of  the  origin  of  dolomite  has  long  occupied  the 
attention  of  geologists  and  many  theories  have  been  advanced 
for  its  formation,  but  no  one  of  these  theories  has  been  widely 
accepted.  Von  Bucli  (l)f  was  the  first  to  seriously  attempt  to 
explain  the  formation  of  the  rock.  As  early  as  1822  in  his 
writings  on  the  dolomite  of  the  Tyrol,  he  ascribed  its  origin  to 
the  action  of  volcanic  vapors,  rich  in  magnesia,  on  limestone, 
and  there  was  some  basis  for  this  belief,  for  the  rocks  are  there 
penetrated  by  augite-porphyry.  Frapolli(2)  and  Durocher  (3) 
later  expressed  similar  views  upon  the  origin  of  the  rock,  and 
Favre  (4),  basing  his  supposition  upon  the  conditions  of  the 
experimental  production  of  dolomite  by  Marignac,  concluded 
that  the  dolomite  of  the  Tyrol  was  formed  by  the  alteration  of 
limestone  beneath  the  sea  at  a  temperature  of  200°  C.  and  at  a 
pressure  of  15  atmospheres,  corresponding  to  a  depth  of  150  to 
200  meters,  by  magnesium  compounds  furnished  by  the  action 
of  sulphurous  and  hydrochloric  acids  of  volcanic  origin  on  the 
lava  of  submarine  melaphyr  eruptions. 

In  1834  Collegno  (5)  pointed  out  the  frequent  association  of 
gypsum  and  dolomite  in  the  St.  Gothard  region  and  regarded 
them  both  as  transformation  products  resulting  from  the  action 
of  magnesium  sulphate  in  surface  waters  or  limestone.  Mor- 
lot(6)  also  favored  such  a  theory  of  origin. 

As  early  as  1836  Beaumont  (7)  ascribed  the  origin  of  dolo¬ 
mite  to  the  alteration  of  limestone  by  circulating  solutions  of 
magnesium  bicarbonate  and,  assuming  that  the  replacement  was 
molecular,  he  calculated  that  the  change  should  be  accompanied 
by  a  decrease  in  volume  of  the  original  rock  to  the  extent  of 
about  12T  per  cent.  Actual  porosity  determinations  by  Mor- 
lot  (8)  on  a  dolomite  sample  from  the  Alps  later  seemed  to  con¬ 
firm  this  prediction. 

In  1843,  A.  W.  Jackson  (9)  suggested  that  ascending  spring- 
water  bearing  magnesium  bicarbonate  might  effect  the  change. 
Nauck,  Haussmann,  Bischof,  Zirkel  and  others,  however, 
subscribed  to  the  view  that  ordinary  circulating  ground  water 
bearing  magnesium  bicarbonate  had  attacked  the  limestone. 

*  The  present  article  is  based  on  a  more  extended  paper  which  constitutes 
a  portion  of  volume  xxv  of  the  Iowa  Geological  Survey.  The  reader  is 
referred  to  this  report  for  details. 

f  For  numbered  references  to  the  literature,  see  the  list  at  the  end  of  this 
article. 


250  VanTuyl — New  Points  on  the  Origin  of  Dolomite. 

Yan  Hise(10)  also  attaches  much  importance  to  dolomitization 
after  the  limestones  emerge  from  the  sea. 

In  1846  Green  (11)  offered  the  suggestion  that  some  dolo- 
mitic  limestones  might  be  formed  by  the  decomposition  of 
olivine  sand  incorporated  in  the  original  limestone  and  the 
recombination  of  the  magnesia  with  the  lime.  He  calls  attem 
tion  to  the  fact  that  olivine  sand,  derived  from  the  action  of  the 
waves  on  lava,  constitutes  an  important  constituent  of  the 
coral-reef  rock  about  the  borders  of  the  Hawaiian  Islands  and 
regards  this  as  significant. 

Dana  (12)  in  1843,  attempting  to  account  for  the  dolomite  of 
the  coral  island  of  Metia,  supposed  that  it  had  been  formed  by 
the  action  of  magnesium  salts  of  heated  seawater  on  limestone. 
Twenty-nine  years  (13)  later  he  expressed  the  view  that  the 
same  dolomite  had  been  formed  in  sea  water  at  ordinary  tem¬ 
peratures  but  perhaps  in  a  contracting  lagoon  where  magnesium 
and  other  salts  were  in  a  concentrated  state.  Sorby(14)  like¬ 
wise  favored  the  theory  of  marine  alteration  and  the  same 
origin  has  been  urged,  either  for  dolomites  in  general  or  in 
special  instances,  by  Yon  Richthofen,  Doelter  and  Hoernes, 
Hoppe-Seyler,  Mojsisovics,  Murray,  Skeats,  and  F.  W.  Pfaff. 
In  support  of  this  theory  are  also  the  observations  of 
Weller  (15)  who,  from  a  faunal  study  of  the  Galena  and 
Niagara  dolomites  of  the  Upper  Mississippi  Yalley,  concludes 
that  they  were  deposited  originally  as  limestones  and  later 
metamorphosed.  More  recently,  Black  welder  (16)  has  also 
advocated  the  replacement  theory  for  the  origin  of  the  Big¬ 
horn  dolomite  of  Wyoming,  but  owing  to  the  very  slight  por¬ 
osity  of  this  rock  he  is  led  to  suggest  that  the  alteration 
proceeded  contemporaneously  with  its  deposition  rather  than 
subsequent  to  its  consolidation. 

F.  W.  Pfaff  (1Y)  believes  that  the  alteration  takes  place  at 
considerable  depth  and  in  concentrated  seas,  but  Phillipi  (18) 
vigorously  controverts  this  view  since  he  has  good  evidence 
that  dolomitization  may  proceed  in  the  open  sea  and  at  shallow 
depths.  Skeats’ (19)  studies  of  the  coral  reefs  of  the  Southern 
Pacific  also  seem  to  show  that  concentration  and  pressure  are 
not  important  factors.  On  the  other  hand,  both  Nadson  and 
Walther(20)  have  suggested  that  bacteria  may  play  an  impor¬ 
tant  part  in  the  alteration. 

Still  other  geologists  have  supported  the  theory  that  dolo¬ 
mite  represents  a  direct  chemical  precipitate  from  the  ocean. 
Bone  (21)  as  early  as  1831  advocated  this  method  of  origin. 
Bertrand-Geslin  (22)  and  Coquand  (23)  were  also  early  sup¬ 
porters  of  this  view.  That  dolomite  can  be  formed  as  a  chem¬ 
ical  precipitate  is  pointed  out  by  Zirkel(24)  who  shows  that 
the  occurrence  of  crystals  of  dolomite  in  veins  and  druses  indi- 


YanTuyl — New  Points  on  the  Origin  of  Dolomite.  251 

cates  its  possible  chemical  deposition  on  a  larger  scale  in 
nature.  Fournet(25)  regarded  the  dolomite  beds  interstratitied 
with  limestone  in  the  Tyrol  as  original  precipitates.  His 
studies  showed  that  the  volcanic  theory  of  Yon  Buch  was  no 
longer  tenable.  Others  who  have  advocated  the  primary  pre¬ 
cipitation  theory  in  one  form  or  another  are  Loretz,  Forch- 
hammer,  Hunt,  Vogt,  Daly,  Linck,  and  Suess. 

As  to  the  nature  and  cause  of  the  reactions  which  have  been 
supposed  to  give  rise  to  the  chemical  precipitation  of  dolomite, 
there  have  been  differences  of  opinion.  Forchhammer  (26) 
attributed  the  reaction  to  the  action  of  calcium  carbonate  of 
spring  water  on  the  magnesium  salts  of  the  sea,  while  Hunt, 
(27)  basing  his  views  on  experimental  evidence,  regarded  dolo¬ 
mite  as  the  product  of  the  action  of  sodium  bicarbonate  on 
the  magnesium  chloride  and  magnesium  sulphate  of  the  sea. 
Linck  (28)  and  Dalv  (29),  on  the  other  hand,  emphasize  the 
importance  of  ammonium  carbonate  furnished  by  decaying 
organisms  on  the  sea  bottom  as  the  precipitating  agent. 

Still  another  primary  theory  is  that  introduced  by  Lesler  (30) 
to  account  for  certain  dolomitic  layers  in  the  “  Calciferous  ” 
limestone  near  Harrisburg,  Pa.  These  lie  believed  to  repre¬ 
sent  ordinary  mechanical  sediments  which  were  deposited  at 
the  time  the  limestone  was  laid  down.  The  clastic  theory  has 
been  adopted  more  recently  by  Phillipi  (31),  who  regards  cer¬ 
tain  impure  dolomites  of  the  Muschelkalk  of  Germany  as 
mechanical  deposits  possibly  derived  from  the  residuum  of 
limestones  low  in  magnesia.  Grabau  (32)  has  concluded  that 
certain  impure  dolomitic  limestones  and  waterlimes  of  the 
Salinan  and  Monroan  series  have  had  a  similar  origin. 

An  entirely  different  theory  of  origin  is  that  which  was  in¬ 
troduced  by  Grand  jean  (33)  in  1844,  to  explain  the  production 
of  the  dolomites  of  the  Lahn  district.  He  assumed  that  by  the 
atmospheric  leaching  of  the  lime  from  an  original  limestone  of 
low  magnesia  content,  a  true  dolomite  might  in  time  result. 
Both  Bischof  and  Hardman  later  demonstrated  the  plausibil¬ 
ity  of  this  theory  experimentally,  and  Hardman  (34)  immedi¬ 
ately  accepted  it  to  explain  the  origin  of  the  Carboniferous 
dolomites  of  Ireland.  In  1895  Hall  and  Sardeson  (35)  applied 
the  same  theory  in  interpreting  the  history  of  the  Lower  Mag¬ 
nesian  series  of  the  Upper  Mississippi  Valley. 

Hogbom  (36)  on  the  other  hand,  regards  surface  leaching  as 
of  minor  importance  and  emphasizes  the  effect  of  marine 
leaching.  He  has  proven  the  reality  of  this  process,  on  a  small 
scale  at  least,  in  the  modern  seas  and  concludes  that  some  dolo¬ 
mites  of  former  periods  may  have  been  formed  in  this  manner. 
Judd  (37)  is  of  the  opinion  that  the  weakly  dolomitic  portions 
of  the  atoll  of  Funafuti  may  be  explained  upon  the  basis  of 

Am.  Jour.  Sci. — Fourth  Series,  Vol.  XLII,  No.  249. — September,  1916. 

17 


252  VanTuyl — New  Points  on  the  Origin  of  Dolomite . 


this  theory  but  regards  the  magnesia  content  of  the  more 
highly  dolomitic  portions  as  having  been  enriched  by  reaction 
with  the  magnesia  of  sea  water. 


Experimental  Evidence. 

On  the  experimental  production  of  dolomite  there  is  a  vol¬ 
uminous  literature.  This  has  been  well  summarized  by  F.  W. 
Pfatf  (38),  and  later  by  Steidtmann  (39).  Dolomite  has  been 
frequently  prepared  artificially  under  conditions  of  high  tem¬ 
perature  or  high  pressure,  or  both,  but  it  has  been  produced 
in  the  laboratory  at  ordinary  temperatures  and  pressures  only 
in  rare  instances,  and  then  in  minute  amounts  and  under  con¬ 
ditions  which  doubtfully  operate  in  nature,  at  least  on  a  large 
scale.  It  must  be  conceded  then  that  these  experiments  fur¬ 
nish  little  evidence  as  to  the  actual  conditions  obtaining  when 
extensive  beds  of  dolomite  are  formed  naturally.  For  the  pur¬ 
pose  of  obtaining  more  accurate  data  on  this  point,  a  series  of 
experiments  was  begun  at  ordinary  temperatures  and  pressures 
early  in  1912.  In  this  series  it  was  attempted  to  simulate 
natural  conditions  as  near  as  they  could  be  estimated,  and  to 
obtain  some  quantitative  measurement  of  the  effect  of  time 
and  of  concentration  in  the  production  of  dolomite.  In  one 
set  of  experiments  it  was  attempted  to  reproduce  the  condi¬ 
tions  which  exist  in  nature  when  limestone  is  altered  to  dolo¬ 
mite  beneath  the  sea  by  solutions  bearing  magnesia.  In  these 
the  effect  of  solutions  of  known  concentration  of  MgCl2  and 
MgS04,  and  of  mixtures  of  the  salts,  both  with  and  without 
the  presence  of  NaCI,  on  powdered  aragonite  was  tried.  The 
concentration  of  the  magnesium  solutions  used  ranged  from 
two  to  ten  times  the  concentration  of  the  magnesia  in  sea 
water.  After  a  period  of  six  months,  residues  from  the  experi¬ 
ments  were  thoroughly  tested  for  dolomite.  The  results  were 
entirely  negative.  No  trace  of  dolomite  could  be  found. 
Careful  re-examination  of  the  residues  after  a  period  of  nearly 
three  years  still  gave  the  same  result.  The  analyses  showed 
that  the  CaC03  had  reacted  slightly  with  the  solutions,  but  no 
MgC03  had  been  deposited.  Apparently  the  soluble  trihy¬ 
drate  of  MgC03  had  been  formed.  It  then  appears  that  dolo¬ 
mite  cannot  be  prepared  artificially  under  these  conditions. 

In  a  second  set  of  experiments  it  was  attempted  to  obtain 
dolomite  as  a  direct  chemical  precipitate  at  ordinary  tempera¬ 
tures  and  pressures.  First  solutions  of  the  bicarbonates  of  cal¬ 
cium  and  magnesium,  after  being  standardized,  were  mixed  in 
molecular  equivalent  proportions  so  as  to  give  the  same  ratio 
of  CaC03  to  MgC03  as  exists  in  normal  dolomite.  The  solu¬ 
tion  was  then  allowed  to  evaporate  spontaneously  during  a 


VcinTuyl — New  Points  on  the  Origin  of  Dolomite.  253 

period  of  one  month.  It  was  noted  that  the  carbonates  came 
down  separately  with  the  CaC03  much  in  advance  of  the 
MgCOs.  The  precipitate  then  contained  only  the  mixed  car¬ 
bonates — no  dolomite  was  formed.  Scheerer  (40)  previously 
obtained  the  same  results  in  a  similar  experiment.  Negative 
results  were  still  obtained  when  a  solution  prepared  as  above 
was  inoculated  with  a  crystal  of  dolomite  and  allowed  to  evapo¬ 
rate.  Nor  could  the  double  carbonate  be  prepared  upon  evapo¬ 
rating  spontaneously  a  solution  of  the  two  carbonates  obtained 
by  the  action  of  carbonated  waters  on  normal  dolomite  even 
when  a  dolomite  crystal  was  introduced  and  a  concentrated 
solution  of  sodium  chloride  and  magnesium  salts  was  added. 

The  experimental  evidence  so  far  obtained,  therefore,  does 
not  suggest  the  circumstances  under  which  large  masses  of 
dolomite  can  be  formed  in  nature  under  ordinary  conditions 
either  by  the  alteration  of  limestone  or  by  chemical  precipita¬ 
tion.  It  is  to  be  regretted  that  a  careful  study  of  the  process 
of  dolomitization  where  it  is  going  on  in  the  seas  to-day  has 
never  been  made.  Such  a  study  would  doubtless  throw  much 
valuable  light  on  the  problem.  It  may  well  be  that  bacteria 
play  an  important  role  in  the  production  of  dolomite  as  sug¬ 
gested  by  Nad  son. 


Field  Evidence. 

Realizing  the  importance  of  careful  field  studies  of  dolomitic 
formations  in  interpreting  the  conditions  of  their  origin,  the 
writer  undertook  a  study  of  the  dolomites  of  the  Upper  Missis¬ 
sippi  Valley  under  the  auspices  of  the  Iowa  Geological  Survey 
during  the  field  season  of  1912.  More  recently  a  grant  from 
the  Esther  Ilerrman  Research  Fund  of  the  New  York  Academy 
of  Sciences  has  made  possible  much  more  extensive  field  studies 
in  the  eastern  and  central  states.  Dolomites  ranging  in  age 
from  the  Cambrian  to  the  Mississippian  have  now  been  exam¬ 
ined  and  many  samples  collected  for  detailed  chemical  and 
petrographic  study.  It  is  possible  to  outline  in  this  paper 
only  some  of  the  more  important  results  obtained. 

It  should  be  pointed  out  that  the  term  dolomite  is  used  here 
in  the  broad  sense  to  include  both  normal  dolomite  and  dolo¬ 
mitic  limestone.  It  is  not  necessary  to  differentiate  between 
these  in  a  discussion  of  their  origin. 

The  field  studies  undertaken  during  the  course  of  this  inves¬ 
tigation  have  alone  furnished  irrefutable  evidence  that  most  of 
the  dolomites  examined,  regardless  of  their  age,  are  replace¬ 
ment  products.  The  following  facts  support  this  contention  : 
(1)  the  lateral  gradation  of  beds  of  dolomite  into  limestone, 
sometimes  very  abruptly;  (2)  the  mottling  of  limestones  by 


254  VanTuyl — New  Points  on  the  Origin  of  Dolomite. 

irregular  patches  of  dolomite  on  the  borders  of  dolomite 
masses  ;  (3)  the  existence  of  remnants  of  unaltered  limestone  in 
dolomite,  and  of  nests  of  dolomite  in  limestone;  (4)  the  irregu¬ 
lar  boundaries  between  certain  beds  of  limestone  and  dolo¬ 
mite  ;  (5)  the  presence  of  altered  oolites  or  fossils  in  many 
dolomites ;  (6)  the  protective  effect  of  shale  beds  :  and  (7)  the 
obliteration  of  structures  and  textures. 

In  some  instances  the  relationship  of  dolomite  to  limestone 
is  such  as  to  indicate  that  the  alteration  was  accomplished  by 
solutions  which  migrated  from  above  downwards  after  the  lime¬ 
stone  was  formed,  or  at  least  in  the  closing  stages  of  its  forma¬ 
tion. 

It  is  an  interesting  fact  that  certain  layers  have  sometimes 
been  passed  over  during  the  dolomitization  of  adjacent  ones, 
and  show  little  or  no  sign  of  alteration.  The  so-called  inter- 
stratification  of  limestone  and  dolomite  cited  by  some  as  evi¬ 
dence  in  favor  of  some  primary  theory  of  origin  is  then,  in 
some  cases  at  least,  rather  a  pseudo-inter-stratification  produced 
by  the  selective  dolomitization  of  an  original  limestone.  Some 
layers  which  have  been  passed  over  have  been  noted  to  be 
coarser  grained  than  the  adjacent  layers  which  have  been 
altered  and  this  would  seem  to  explain  their  greater  resisting 
power.  At  times,  however,  the  unaltered  layers  do  not  appear 
to  differ  markedly  from  the  altered  ones.  The  phenomenon  is 
then  difficult  to  account  for.  Normally  the  contact  lines 
between  such  interbedded  layers  of  limestone  and  dolomite  are 
fairly  regular  and  definite,  but  in  some  instances  they  are 
known  to  be  very  irregular  and  may  even  simulate  irregular 
contacts  produced  by  disconformity.  A  remarkable  example 
of  a  pseudo-disconformity  produced  by  uneven  selective  dolo¬ 
mitization  has  been  observed  in  the  St.  Louis  limestone  near 
Farmington,  Iowa.  Here  a  bed  of  altered  limestone  is  found 
resting  very  irregularly  on  a  bed  of  dolomite.  The  two  beds 
are  very  different  physically  and  might  readily  be  taken  at  first 
sight  for  two  distinct  formations,  but  when  the  contact  is 
traced  laterally  for  a  short  distance  the  lower  bed  loses  its  dolo- 
mitic  character  and  passes  into  a  limestone  very  similar  to  and 
continuous  with  the  bed  above. 

Another  striking  relationship  of  limestone  to  dolomite  is 
exhibited  in  a  certain  layer  of  an  interbedded  series  of  lime¬ 
stones  and  dolomites  of  the  Beekmantown  in  the  old  Walton 
Quarry  near  Harrisburg,  Pa.  The  beds  dip  south  here  at  an 
angle  of  30°.  The  layer  in  question  is  represented  by  dolo¬ 
mite  six  feet  in  thickness  in  the  upper  part  of  the  quarry  face 
and  on  each  side  of  it  appear  good  limestone  layers.  Now  in 
the  lower  part  of  the  quarry  the  lower  half  of  this  layer  passes 
abruptly  into  limestone  and  continues  to  the  quarry  floor  as 


VanTuyl — New  Points  on  the  Origin  of  Dolomite.  255 

two  distinct  layers  each  3  feet  thick.  Samples  of  the  dolo¬ 
mite  at  the  point  where  it  passes  into  limestone  yielded  18*1 
per  cent  of  MgC03  while  the  limestone  itself  yielded  only  0*83. 

It  will  be  noted  that  in  the  above  instances  the  gradation 
of  limestone  into  dolomite  is  abrupt,  but  in  many  cases  the 
gradation  takes  place  through  transition  zones  of  limestone 
mottled  with  dolomite.  There  can  be  no  doubt  but  that  these 
mottled  limestones  represent  an  incipient  stage  in  the  process 
of  dolomitization  and  it  is  believed  that  many  dolomites  have 
passed  through  such  a  stage  in  the  progress  of  their  formation. 
In  most  cases  the  phenomenon  of  mottling  appears  to  be  of 
purely  inorganic  origin,  having  resulted  from  a  process  of  dolo¬ 
mitization  which  began  at  certain  favorable  centers  and  spread 
outwards.  In  some  cases,  however,  it  has  been  produced  by 
the  selectine  alteration  of  areas  suggesting  algae  and  fucoids  in 
the  limestone  first,  and  the  spreading  out  of  the  dolomite  from 
these  as  nuclei.  The  Tribes  Hill  limestone,  as  developed  at 
Canajoharie,  New  York,  furnishes  an  excellent  illustration  of 
the  mottling  produced  by  the  latter  method.  All  stages  of 
mottling  from  altered  fucoid-like  markings  to  a  rock  uniformly 
dolomitic  may  be  traced  in  this. 

It  has  been  observed  that  the  spreading  of  dolomitization 
from  certain  centers  in  a  limestone  may  give  rise  to  mottling 
on  a  large  scale  if  these  centers  be  few  and  far  apart.  For 
example  there  is  a  conspicuous  bed  of  dolomite  pseudo-bowlders 
in  the  St.  Louis  limestone  at  Alton,  Ill.,  which  appears  to  have 
been  formed  entirely  in  this  manner.  These  bowlder-like 
masses  range  from  a  few  inches  up  to  six  feet  in  diameter  and 
contain  32’39  per  cent  of  MgC03  while  the  limestone  matrix 
bears  only  3*39.  That  they  were  formed  in  place  is  clearly 
indicated  by  the  fact  that  the  contact  of  the  bowlders  with  the 
limestone  matrix  is  occasionally  gradational  and  that  the  strat¬ 
ification  lines  of  the  limestone  may  at  times  be  traced  directly 
through  the  bowlders.  In  a  layer  of  limestone  a  few  feet 
above  the  bowlder  bed  here  a  similar  process  of  local  dolomiti¬ 
zation  has  given  rise  to  the  development  of  irregular  lenses  of 
dolomite. 

If  has  often  been  noted  during  the  course  of  the  field  studies 
that  many  dolomites  known  to  be  of  secondary  origin  show 
little  or  no  evidence  of  shrinkage  and  porosity  determinations 
have  since  shown  that  the  transformation  of  a  limestone  to 
dolomite,  even  subsequent  to  its  deposition,  need  not  neces¬ 
sarily  be  accompanied  by  a  decrease  in  volume  as  pointed  out 
by  Beaumont  and  consistently  adhered  to  by  later  writers  on 
the  subject.  It  seems  probable,  therefore,  that  the  replace¬ 
ment  may  proceed  at  times  according  to  the  law  of  equal 
volumes  as  enunciated  by  Lindgren(41)  and  that  the  inter- 


256  VcinTuyl — New  Points  on  the  Origin  of  Dolomite. 

change  need  not  be  molecular.  In  view  of  this  fact  compact 
dolomites  showing  no  shrinkage  effects  can  no  longer  be 
regarded  as  primary. 

Further  studies  will  doubtless  show  that  considerable  shrink¬ 
age  effects  produced  by  dolomitization  are  not  common.  It  is 
believed  that  many  vesicular  dolomites  have  resulted  from 
atmospheric  leaching  long  after  their  formation. 

Petrographic  Evidence. 

The  microscopic  study  of  many  thin  sections  of  dolomitic 
limestone  has  not  only  further  amplified  and  strengthened  the 
field  evidence  but  has  also  thrown  new  light  upon  the  details 
of  the  process  of  alteration.  By  employing  microchemical 
tests  it  lias  been  possible  to  distinguish  between  calcite  and 
dolomite  in  the  sections  and  make  clear  the  most  intimate  rela¬ 
tionships  of  the  two  minerals.  It  should  be  stated,  however, 
that  these  tests  furnish  no  reliable  guide  to  the  exact  amount 
of  magnesia  in  the  rock,  for  crystals  containing  less  than  25  per 
cent  of  MgC03  may  behave  essentially  like  normal  dolomite. 
But  this  is  to  be  regarded  in  truth  as  a  distinct  advantage,  for 
alterations  of  only  a  slight  degree  are  indicated  as  well  as  the 
more  marked  ones. 

It  must  be  admitted  that  if  dolomite  has  a  diverse  mode  of 
origin  the  microscope  fails  to  reveal  it.  Careful  examination 
of  every  variety  of  dolomite  fails  to  show  any  positive  evidence 
in  favor  of  either  the  primary  chemical  or  the  clastic  theory  of 
origin.  On  the  other  hand,  there  is  abundant  evidence  in 
favor  of  the  alteration  theory.  It  is  true  that  certain  dolo¬ 
mites,  whose  origin  is  not  certainly  known  from  their  field 
relations,  possess  an  extremely  fine  and  uniform  texture,  and 
this  feature  has  in  fact  led  Daly  (42)  to  believe  that  these  repre¬ 
sent  original  chemical  precipitates.  In  order  to  test  the 
validity  of  this  argument  the  finest  grained  dolomite  of 
unknown  origin  encountered  by  the  writer  in  these  studies  was 
compared  with  the  finest  grained  dolomite  of  known  secondary 
origin.  For  example  the  Jefferson  City  dolomite  of  the  Ozark 
region,  whose  mode  of  formation  is  not  definitely  known,  pos¬ 
sesses  unusually  dense  and  compact  layers  which  are  seen 
under  the  microscope  to  be  made  up  of  minute  granules  the 
majority  of  which  are  below  '003mm  in  diameter,  some  meas¬ 
uring  only  *001ram.  The  granules  are  suggestive  of  an  original 
chemical  precipitate.  The  strength  of  this  interpretation  is 
weakened,  however,  by  the  fact  that  a  dolomite  of  known 
secondary  origin  has  been  found  in  the  Middle  Devonian  of 
Iowa  which  is  equally  as  fine-grained.  The  latter  dolomite  has 
resulted  from  the  alteration  of  a  dense,  lithographic-like  lime¬ 
stone  with  the  approximate  retention  of  the  original  texture. 


VanTuyl — New  Points  on  the  Origin  of  Dolomite.  257 

As  regards  the  possibility  that  some  dolomites  may  be  of 
clastic  origin,  none  has  been  found  which  exhibits  any  signs  of 
clastic  structure.  But  that  the  original  structure  might  have 
been  obliterated  during  recrystallization  is  easily  conceivable 
in  rocks  of  this  type. 

Turning  now  to  the  dolomites  which  from  their  field  rela¬ 
tions  are  known  to  be  secondary  after  limestone  we  have  much 
more  definite  data.  Indeed  in  these,  by  virtue  of  the  fact  that 
the  alteration  has  frequently  been  halted  before  it  proceeded 
to  completion,  we  are  often  able  to  trace  all  stages  of  dolomiti- 
zation  from  a  limestone  showing  only  incipient  alteration  to  a 
good  dolomite.  Thus,  it  is  possible  to  describe  the  steps 
normally  passed  through  during  the  transformation  of  a  lime¬ 
stone  to  dolomite. 

So  far  as  the  testimony  of  the  microscope  goes  the  fine¬ 
grained  limestones  are  more  susceptible  to  alteration  than  the 
coarser-grained  ones,  a  fact  which  is  in  keeping  with  the  laws 
of  chemistry.  The  evidence  also  indicates  that  the  alteration 
may  not  proceed  in  exactly  the  same  manner  in  the  two  types 
of  limestone. 

The  alteration  of  fine-grained  compact  limestones  seems  to 
be  accompanied  normally  by  a  notable  increase  in  size  of  grain. 
Usually  the  diameter  of  the  dolomite  crystals  formed  is  many 
times  as  great  as  that  of  the  original  calcite  grains.  But  in 
rare  cases,  such  as  that  of  the  dense  Middle  Devonian  dolomite 
referred  to  above,  the  original  structure  and  texture  seems  to 
be  approximately  retained.  In  the  dolomitization  of  such  fine¬ 
grained  limestones  the  replacement  frequently  begins  at  many 
centers  throughout  the  rock  and  spreads  outwards  from  these, 
or  if  the  rock  possesses  fine  stratification  .the  replacement  may 
follow  closely  these  original  lines  of  weakness  in  the  early 
stages.  In  those  cases  where  the  alteration  begins  at  certain 
centers  and  spreads  out  from  these,  fucoid-like  markings  occa¬ 
sionally  serve  as  the  nuclei  as  in  the  case  of  the  Tribes  Hill 
limestone.  But  as  a  general  rule  no  organic  influence  is  noted. 
Normally  the  limestone  is  altered  uniformly  in  the  process  of 
spreading  from  the  dolomite  centers,  but  it  cannot  be  said  that 
it  is  altered  completely,  for  the  dolomite  patches  often  possess 
less  than  twenty  per  cent  of  MgCG3.  Small  remnants  of  lime¬ 
stone,  however,  may  occasionally  escape  alteration  and  become 
incorporated  in  the  dolomite  patches.  The  boundary  between 
the  limestone  and  the  spreading  dolomite  area  may  or  may  not 
be  abrupt.  When  abrupt,  the  rock  may  assume  the  appearance 
of  a  breccia  and  the  term  “  pseudo-breccia  ”  may  aptly  be 
applied.  When  the  boundary  is  gradational,  on  the  other 
hand,  rhombohedrons  of  dolomite,  variable  in  size  but  usually 
nearly  perfect  in  their  development,  are  disseminated  through 


258  VanTuyl — New  Points  on  the  Origin  of  Dolomite. 

the  limestone  a  short  distance  in  advance  of  the  main  dolomite 
area.  As  the  replacement  proceeds  the  dolomite  areas  grow 
larger  and  larger  and  eventually  meet  and  become  confluent 
thereby  giving  rise  to  a  rock  which  is  uniformly  dolomitic. 
Further  addition  of  magnesia  may  then  take  place  by  altering 
the  rock  more  completely. 

In  the  coarse-grained  limestones,  especially  those  which  were 
originally  coarse-grained,  such  as  the  crinoidal  limestones,  on 
the  other  hand,  mottling  does  not  seem  to  be  the  rule  in  the 
early  stages  of  alteration.  In  these  the  replacement  appears  to 
affect  the  matrix  first  and  to  spread  rapidly  through  the  rock. 
The  coarser  grains  are  next  affected,  being  broken  down  into 
aggregates  of  small  dolomite  grains.  In  the  end  a  coarse¬ 
grained  limestone  may  be  changed  over  into  a  uniformly  fine¬ 
grained  one. 

In  the  dolomitization  of  limestones  of  both  types  the  calcar¬ 
eous  skeletons  of  organisms  appear  in  most  cases  to  successfully 
withstand  alteration  and  these,  owing  to  their  greater  solubility 
than  dolomite,  are  then  removed  to  leave  molds  by  a  process 
of  atmospheric  leaching  when  the  formation  passes  into  the 
belt  of  weathering. 


Conclusions. 

Considering  all  the  evidence,  it  seems  probable  that  the 
great  majority  of  our  dolomites  had  their  inception  in  the 
alteration  of  limestones.  It  will  not  be  denied,  however,  that 
some  dolomitic  formations  of  minor  importance  may  have  had 
a  different  origin.  For  instance,  some  impure  dolomitic  lime¬ 
stones  associated  with  shales  very  probably  represent  original 
clastic  deposits  which  have  not  suffered  any  alteration  what¬ 
ever,  and  there  is  some  reason  for  believing  that  certain  dolo¬ 
mitic  limestones  high  in  siliceous  material,  such  as  the  Silurian 
waterlimes  of  New  York  State,  may  have  had  a  similar  origin. 

The  importance  of  marine  and  surface  leaching  in  increasing 
the  magnesia  content  of  limestones  originally  low  in  magnesia 
should  likewise  not  be  overlooked.  There  can  be  no  doubt 
that  this  process  has  greatly  enriched  the  more  vesicular  dolo¬ 
mitic  limestones  in  magnesia.  But  the  leaching  theory  does 
not  explain  the  ultimate  source  of  the  magnesia.  It  merely 
shows  how  the  magnesia  content  of  a  limestone  originally  low 
in  this  constituent  can  be  enriched. 

To  return  now  to  the  dolomites  which  have  resulted  from 
the  alteration  of  limestone,  there  are  many  reasons  for  believ¬ 
ing  that  the  more  extensive  of  these  have  all  been  formed 
beneath  the  sea,  and  that  dolomitization  affected  by  ground 
water  is  only  local  and  very  imperfect.  Some  of  the  features 


VanTuyJ — New  Points  on  the  Origin  of  Dolomite.  259 

which  lend  weight  to  this  view  are  as  follows :  (1)  The  dolo¬ 
mite  areas  of  mottled  limestones  are  believed  to  have  undergone 
recrystallization  at  the  same  time  as  the  associated  limestone 
areas  as  suggested  by  the  occasional  development  of  zonal 
growths  of  calcite  and  dolomite.  (2)  In  imperfectly  altered 
limestones  the  dolomite  is  seen  to  follow  original  lines  of  weak¬ 
ness  rather  than  secondary  structures  such  as  joints  or  fractures. 
(3)  In  most  cases  of  mottling  the  dolomitization  appears  to 
have  progressed  uniformly  as  we  should  expect  it  to  in  an 
uncrystallized  rock,  rather  than  to  have  progressed  by  forming 
veinlets  and  stringers  in  the  early  stages.  (4)  The  existence  of 
perfect  rhombs  of  dolomite  in  many  imperfectly  altered  lime¬ 
stones  suggests  that  the  latter  had  not  yet  solidified  when  the 
dolomite  rhombs  were  formed.  (5)  The  widespread  extent 
and  nearly  uniform  composition  of  many  dolomites  indicates 
that  they  must  have  been  formed  by  an  agent  capable  of  oper¬ 
ating  uniformly  over  wide  areas.  (6)  An  adequate  source  of 
magnesia  for  transforming  extensive  limestone  formations  into 
dolomite  is  found  only  in  the  sea  which  contains  many  times  as 
much  of  this  constituent  as  ordinary  ground  water.  (7)  Many 
dolomites  are  directly  and  regularly  overlain  by  pure  limestone 
formations  or  by  thick  shale  beds,  proving  that  they  must  have 
been  formed  before  these  overlying  beds  were  deposited. 

Some  dolomites  of  minor  importance,  such  as  those  associ¬ 
ated  with  ore  deposits  and  probably  most,  if  not  all  of  those 
related  to  fractures  (vein  dolomites),  must  have  been  formed 
through  the  agency  of  ground  water.  But  in  general,  ground 
water  is  incapable  of  carrying  dolomitization  far.  Study  of 
analyses  of  ground  water,  and  of  river  water,  shows  these  to 
be  uniformly  low  in  magnesia,  this  constituent  normally  being 
greatly  exceeded  in  amount  by  lime.  How,  then,  could  such 
waters  dolomitize  limestone  when  they  already  contain  lime 
far  in  excess  of  magnesia  ?  The  law  of  mass  action  speaks 
strongly  against  ordinary  ground  water  being  able  to  accom¬ 
plish  extensive  dolomitization.  In  the  case  of  mineral  springs 
and  the  mineralizing  solutions  which  are  related  to  ore 
deposition,  however,  it  is  conceivable  that  magnesia  might  be 
present  in  sufficient  proportions  to  accomplish  local  dolomitiza¬ 
tion  and  doubtless  most  vein  dolomites  have  been  so  formed. 

University  of  Illinois, 

Urbana,  111. 


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260  YanTuyl — New  Points  on  the  Origin  of  Doloinite. 

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